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LTC3615

3615f

Low Supply Operation

The LTC3615 is designed to operate down to an input

supply voltage of 2.25V. An important consideration

at low input supply voltages is that the RDS(ON) of the

P-channel and N-channel power switches increases by

50% compared to 5V. The user should calculate the

power dissipation when the LTC3615 is used at 100%

duty cycle with low input voltages to ensure that thermal

limits are not exceeded.

Slope Compensation and Inductor Peak Current

Slope compensation provides stability in current mode

constant-frequency architectures by preventing subhar-

monic oscillations at duty cycles greater than 50%. The

LTC3615 implements slope compensation by adding a

compensation ramp to the inductor current signal.

Short-Circuit Protection

Thepeakinductorcurrentatwhichthecurrentcomparator

shuts off the top power switch is controlled by the voltage

on the ITH pin.

If the output current increases, the error amplifier raises

the ITH pin voltage until the average inductor current

matches the new load current. In normal operation, the

LTC3615 clamps the maximum ITH pin voltage at ap-

proximately 1.05V which corresponds to about 5A peak

inductor current.

When the output is shorted to ground, the inductor current

decays very slowly during a single switching cycle. The

LTC3615 uses two techniques to prevent current runaway

from occurring:

1. If the output voltage drops below 50% of its nominal

value, the clamp voltage at pin ITH is lowered, causing

the maximum peak inductor current to lower gradu-

ally with the output voltage. When the output voltage

reaches 0V, the clamp voltage at the ITH pin drops to

40% of the clamp voltage during normal operation. The

short-circuit peak inductor current is determined by the

minimum on-time of the LTC3615, the input voltage

and the inductor value. This foldback behavior helps

in limiting the peak inductor current when the output

is shorted to ground. It is disabled during internal or

external soft-start and tracking up/down operation (see

the Applications Information section).

2. If the inductor current of the bottom MOSFET increases

beyond 6A typical, the top power MOSFET will be held

off and switching cycles will be skipped until the induc-

tor current reduces.

operaTion

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